Integral power steering is today accepted as the standard in the industry for steering wheeled vehicles. In an integral power steering unit, a piston operates by sliding within a cylindrical with a rack on the piston operating an output gear and pitman arm to shift the steering linkage as required.
One of the most successful power steering gears and design for a piston for the gear is described and claimed in the R. H. Sheppard U.S. Pat. No. 3,092,083, entitled "Piston For Power Steering System." This highly successful basic design has the highest efficiency and reliability, coupled with the lowest manufacturing cost in the industry today. It is particularly adapted to large vehicles, where relatively large steering forces are required.
In the Sheppard-type integral power steering gear unit, the pressurized hydraulic fluid is fed into the unit through an inlet slot on the side of the piston opposite from the output rack that engages the output gear. Under dynamic conditions of the unit, the hydraulic pressure acting on the inlet slot side of the piston offsets to some degree the countering force of the rack and pinion engagement. This gives the greatest efficiency, since with the piston at least partially balanced within the cylinder, minimum frictional forces are at work attempting to retard the movement of the piston.
Over the years, several proposals have been made for reducing friction further, especially in the instance where the piston is initially being moved from a static position. In this mode, of course static friction, increased by the heavy load from the hydraulic pressure adjacent the inlet slot, must be overcome in order to allow the piston to translate and move the steering linkage in the appropriate direction. Low friction condition in this mode of operation is important since the responsiveness of the gear, that is the ability of the piston to "break loose" with minimum force on the steering wheel, is a primary criterion governing the efficiency of a steering gear.
In the area of responsiveness, the Sheppard gears have excelled over the years. Continuous improvements have been made prior to my invention. For example, in U.S. Pat. No. 4,088,063 of R. H. Sheppard entitled "Power Steering Gear with Short Piston and Fluid Bearings," and which is subject along with the '083 patent to an exclusive license to the same company as is exclusively licensed under the present invention, one approach is to provide lubricating grooves along the piston adjacent these areas of potentially greatest friction or wear, that is around the output rack. This design has greatly enhanced the responsiveness of steering gears, since the direction of lubrication of these critical areas allows less static friction, and consequently less wear on the cylinder. This concept has been adopted in the newest generation of gears and has attained wide acceptance as a major step forward in the field. This approach of providing fluidized bearings on the surface of the piston, has allowed trucks previously unsteerable to be steered with relatively small steering gears, and in some instances even at comparatively reduced hydraulic pressures.
I have discovered however that there is one drawback to this design. By providing the lubrication grooves adjacent the output rack, along the sides of the piston, and adjacent the ends, there is some increased leakage in the steering gear unit, particularly under heavy loads where the pressure of the pump is increased to its highest levels. This leakage is caused by the fact that the oil in the grooves is able to squeeze out between the piston and the cylinder into the low pressure side of the steering gear unit. This means that there is under some circumstances, such as a truck in heavy mud or severe parking maneuvers, some loss of pressure that could otherwise be used to advantage to move the piston. Also, there is increased flow from the pump required during all modes of operation as some oil inevitably squeezes out between the piston and the cylinder wall to the low pressure side.